The authors develop a model and define performance measures for a replicated data system that makes use of a quorum-consensus algorithm to maintain consistency. They consider two measures: the proportion of successfully completed transactions in systems where a transaction aborts if data is not available, and the mean response time in systems where a transaction waits until data becomes available. Based on the model, the authors show that for some quorum assignment there is an optimal degree of replication beyond which performance degrades. There exist other quorum assignments which have no optimal degree of replication. The authors also derive optimal read and write quorums which maximize the proportion of successful transactions.<>